The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus in which a digital image is produced by carrying out an image processing appropriate for the characteristics of an input image supplied from a scanner, so that the digital image is output from a printer. This apparatus can be applied to digital copiers, facsimile machines or the like.
In digital copiers or the like, it is desirable to reproduce a high quality digital image from an input image supplied from a scanner no matter what the characteristics of the original image are. In order to reproduce a high quality digital image by means of a printer, it is necessary to select an image processing (or filtering) appropriate for the input image and to produce the digital image by carrying out the image processing for the input image. With the recent development of digital image processing technology, it has become possible to reproduce a very fine image at a quality level compatible with the quality level of the analog image processing without the need for expensive hardware.
However, there are some problems in the digital image processing. Generally, input images to be processed by an image processing apparatus are classified into character images, continuous-tone images (photographic images) and mesh images (printed matters). When a mesh image is processed through a filtering process, an image reproduced from such an input image may have moire due to interference of the sampling intervals and the dot intervals. When a composite image in which a mesh image and a character image coexist is processed, the reproduced image may have undesired dots in a character region and the edges of characters may become faded.
There is a known method for eliminating the above mentioned problems. In this method, it is detected whether a pixel of the input image constitutes a part of a mesh image or a part of a character image before a digital signal processed through a filtering process for the pixel is output to the printer. When the pixel is detected to be in a mesh region, an image signal processed through a smoothing filter is selected, and the signal is output to the printer. When the pixel is detected to be in a character region (also called an edge region), an image signal processed through an edge sharpening filter is selected, and the signal is output to the printer.
However, the type of the filtering process used by the above mentioned method is fixed regardless of the characteristics of the input image. When a composite image in which a character image and a mesh image coexist is processed, it is likely that the edges reproduced from the mesh image will become excessively sharp and the edges reproduced from the character image will become imperceptible due to the filtering process type which is inappropriate for the input image.
There is another method for eliminating the previously mentioned problems. In this method, an edge quantity of each pixel is detected by an edge quantity detector, and, when the edge quantity falls in an intermediate range between the maximum limit and the minimum limit, an image signal processed for the pixel through a smoothing filter and an image signal processed for the pixel through an edge sharpening filter are mixed in accordance with a predetermined mixing factor so that the derived signal is output to the printer.
However, the intermediate range and the mixing factor used in the above mentioned method are preset and cannot be adjusted in accordance with the characteristics of the input image. If the intermediate range and the mixing factor are preset to values appropriate for processing a mesh image, the edges reproduced from a character image (especially, fine lines or edges of a small character) become faded. If the intermediate range and the mixing factor are preset to values appropriate for processing a character image, the edges reproduced from the mesh image become excessively sharp. Therefore, in the above conventional image processing method, it is difficult to reproduce an image with clear edges no matter what the characteristics of the input image are.